This invention relates to a method for producing ammonium paratungstate (APT) from cemented tungsten carbide by electrodissolution. Use of this method avoids the use of processes which involve dissolution of the tungsten in alkali or the use of expensive reagents.
Ammonium paratungstate is normally prepared by any one of several well known techniques. One technique involves the dissolution of tungstic acid in ammonium hydroxide to produce ammonium tungstate followed by filtration and evaporation to crystallize ammonium paratungstate. Another technique involves addition of ammonium chloride to sodium tungstate solution to precipitate ammonium paratungstate and leave sodium chloride in solution. Still another technique involves the extraction of a metatungstate species with an organic extracting agent followed by stripping with ammonium hydroxide to produce ammonium tungstate which is evaporated to produce ammonium paratungstate crystals. Another technique involves addition of ammonium hydroxide to ammonium metatungstate solution to precipitate ammonium paratungstate crystals.
The disadvantages of these processes are that they all require preparation of ammonium tungstate by dissolution or oxidation of the tungsten starting material. Tungsten ores, tungsten scrap, or tungsten carbide scrap must be digested in strong alkali directly or first oxidized to tungstic oxide and then digested in strong alkali. The alkali is usually sodium hydroxide. This produces sodium tungstate. To produce ammonium paratungsten the sodium must be eliminated by precipitation of tungstic acid by the addition of acid, which is usually hydrochloric acid. The resulting tungstic acid is then washed until it is free of sodium, which is difficult to do because tungstic acid becomes colloidal during washing, making it very difficult to wash and to handle. The addition of ammonium chloride to sodium tungstate requires the preparation of the sodium tungstate and the yield of ammonium paratungstate is usually low, and the ammonium paratungstate contains sodium.
The extraction of a tungsten species, using an organic extracting agent requires a large investment in equipment and again requires preparation of sodium tungstate for extraction. The preparation of ammonium paratungstate from metatungstate is very expensive since metatungstate is usually prepared from ammonium paratungstate.